As is well-known, soluble or substrate fixed finely distributed metals, metal colloids and metal clusters are valuable catalysts in organic and inorganic chemistry as well as in electrochemistry (fuel cells) [G. Schmid, Clusters and Colloids, VCH, Weinheim 1994; J. P. Fackler, Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum Press, New York 1990; B. C. Gates, L. Guczi, H. Kn ozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; S. C. Davis, K. J. Klabunde, Chem. Rev. 82 (1982) 153]. This involves the reduction of metal salts by reducing agents, such as hydrogen, alcohol, formaldehyde, hydrazine, alkali metals, anthracene activated magnesium, or boron hydrides. The synthesis often employs stabilizers which prevent undesired formation of metal powders. These include ligands (e.g. phenanthroline derivatives), polymers (e.g. polyvinylpyrrolidone), and surface-active agents (e.g. tetra-alkylammonium salts) [see for instance: G. Schmid, B. Morun, J.-O. Malm, Angew. Chem. 101 (1989) 772; Angew. Chem., Int. Ed. Engl. 28 (1989) 778; M. N. Vargaftik, V. P. Zagorodnikov, I. P. Stolarov, I. I. Moiseev, J. Mol. Catal. 53 (1989) 315; J. S. Bradley, J. M. Milar, E. W. Hill, J. Am. Chem. Soc. 113 (1991) 4016; F. Porta, F. Ragaini, S. Cenini, G. Scari, Gazz. Chim. Ital. 122 (1992) 361; H. B onnemann, W. Brijoux, R. Brinkmann, E. Dinjus, T. Joussen, B. Korall, Angew. Chem. 103 (1991) 1344; Angew. Chem., Int. Ed. Engl. 30 (1991) 1312; M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf. 5 (1982) 209; M. Boutonnet, J. Kizling, R. Touroude, G. Maire, P. Stenius, Appl. Catal. 20 (1986) 163; N. Toshima, T. Takashashi, H. Hirai, Chem. Lett. 1985 1245; K. Meguro, M. Toriyuka, K. Esumi, Bull. Chem. Soc. Jpn. 61 (1988) 341; N. Toshima, T. Takashashi, Bull. Chem. Soc. Jpn. 65 (1992) 400; J. Blum, Y. Sasson, A. Zoran, J. Mol. Catal. 11 (1981) 293; N. Satoh, K. Kimura, Bull. Chem. Soc. Jpn. 62 (1989) 1758]. Sometimes metal vaporization is used [G. Schmid, Clusters and Colloids, VCH, Weinheim 1994; J. P. Fackler, Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum Press, New York 1990; B. C. Gates, L. Guczi, H. Kn ozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; S. C. Davis, K. J. Klabunde, Chem. Rev. 82 (1982) 153]. Drawbacks of these methods are, inter alia, (1) the high costs of metal vaporization and of some reducing agents; (2) partial or undesired formation of metal powders; (3) tedious separation procedures for the purification of the metal clusters or colloids; (4) contamination by partial incorporation of reducing agents (e.g. hydrogen or boron); (5) lack or limitation of facilities for controlling the particle size. Specific and simple control of particle size while synthesis and isolation are simple would just be a large progress, however, all the more, since the catalytic properties of metal colloids and metal clusters depend on particle size [A. Duteil, R. Queau, B. Chaudret, R. Mazel, C. Roucau, J. S. Bradley, Chem. Mater. 5 (1993) 341].
Drawbacks of the above mentioned methods are, inter alia, the high costs of some reducing agents; tedious separation of by-products; impure products from undesired partial incorporation of reducing agents (e.g. hydrogen or boron); and/or lack or limitation of facilities for controlling the particle size.
It is known that in conventional metal powder production, electrochemical processes are also used wherein use is made either of anodic dissolution with subsequent reduction at the cathode or of reduction at the cathode of metal salts employed [N. Ibl, Chem. Ing.-Techn. 36 (1964) 601]. These methods are inexpensive and often clean with respect of the formation of by-products (R. Walker, A. R. B. Sanford, Chem. Ind. 1979, 642; R. Walter, Chem. Ind. 1980, 260). This involves the use of aqueous electrolytes which in most cases comprise sulfuric acid. Although metals and alloys of different morphologies can be prepared in this way, one drawback is the concomitant formation of metal hydrides through H.sub.2 formation at the cathode which is frequently observed [N. Ibl, G. Gut, M. Weber, Electrochim. Acta 18 (1973) 307]. The major drawback, however, is the fact that to date the preparation of soluble nanostructured colloids in the range of up to 30 nm has not been accomplished. Rather, deposition of metal powder in the form of large crystallites in the nm or .mu.m range occurs as a rule.